The degree of integration of integrated circuits is becoming much higher, and the photolithography for superfine patterns having a line width of half micron or below is beginning to be required in the production of semiconductor substrates for VLSI and the like. In order to meet such a requirement, the wavelengths used in an exposure apparatus for photolithography are becoming shorter, and nowadays the use of far ultraviolet light in shorter wavelength region, or excimer laser beams (XeCl, KrF, ArF), is being investigated.
The novolak-naphthoquinone diazide resist is inadequate to be used in the pattern formation by the photolithography in the far ultraviolet region. This is because both novolak resin and naphthoquinone diazide compound have strong absorption in such a wavelength region to inhibit the light from reaching to the bottom of the resist; as a result, the resist has low sensitivity and can merely provide a tapered pattern profile.
For instance, the chemical amplification resist compositions disclosed in U.S. Pat. No. 4,491,628 and European Patent 0,249,139 are one means for solving the above-described problem. The chemical amplification-utilized positive resist composition is a composition designed such that it generates an acid in the exposed part upon irradiation with an actinic ray, such as far ultraviolet light, and causes a reaction using the acid as catalyst to make a difference of solubility in a developer between the irradiated part and the non-irradiated part. By virtue of this solubility difference, a pattern can be formed on a substrate coated with such a composition.
In general, the chemical amplification resist is roughly divided into three types, the so-called two-component, two point five-component and three-component types. The chemical amplification resist of two-component type is a combination of a compound capable of generating an acid by photolysis (herein after referred to as a "photoacid generator") with a binder resin. The binder resin combined with such a compound has, in a molecule, groups capable of being decomposed by the action of an acid to increase the solubility of resin in an alkali developer (which are hereinafter referred to as "acid-decomposable groups"). The resist of two point five-component type further contains a low molecular weight compound having an acid-decomposable group in addition to the above two components. The resist of three-component type contains a photoacid generator, an alkali-soluble resin and the above low molecular weight compound.
Examples of a combination of an photoacid generator with a resin capable of changing its solubility by the action of an acid, which is used for the resist of two- or two point five-component type, include the combination of a photoacid generator with acetal or an O, N-acetal compound (JP-A-48-89003, wherein the term "JP-A" means an unexamined published Japanese patent application"), the combination of a photoacid generator with an orthoester or amidoacetal compound (JP-A-51-120714), the combination of a photoacid generator with a polymer having acetal or ketal groups in its main chain (JP-A-53-133429), the combination of a photoacid generator with an enol ether compound (JP-A-55-12995), the combination of a photoacid generator with an N-acyliminocarbonic acid compound (JP-A-55-126236), the combination of a photoacid generator with a polymer having orthoester groups in its main chain (JP-A-56-17345), the combination of a photoacid generator with a tertiary alkyl ester compound (JP-A-60-3625), the combination of a photoacid generator with a silyl ester compound (JP-A-60-10247), the combinations of a photoacid generator with tertiary or secondary carbon-containing (e.g., t-butyl, 2-cyclohexenyl) esters or carbonate compounds (e.g., JP-A-63-250642, Polym. Eng. Sci., vol. 23, p. 1012 (1983); ACS. Sym., vol. 242, p. 11 (1984); Semiconductor World, the November number, p. 91 (1987); Macromolecules, vol. 21, p. 1475 (1988); and SPIE, vol. 920, p. 42 (1988)), and the combinations of a photoacid generator with silyl ether compounds (JP-A-60-37549 and JP-A-60-121446) Those combinations have a quantum yield greater than 1 in principle, so that they exhibit high sensitivity.
As described above, the chemical amplification resist is a photoresist suitable for exposure to ultraviolet light or far ultraviolet light. Further, it is necessary for the resist to meet the requirements for characteristics from the viewpoint of practical use. In the case of using a KrF excimer laser beam of 248 nm as exposure light, for instance, the resist compositions comprising a hydroxystyrene polymer, which shows only slight absorption of the laser beam, into which acetal or ketal groups are introduced as protective groups, have been proposed in, e.g., JP-A-2-141636, JP-A-2-19847, JP-A-4-219757 and JP-A-5-281745. In addition, the compositions which are similar to the above, except that t-butoxycarbonyloxy or p-tetrahydropyranyloxy groups are introduced thereinto as acid-decomposable groups, have been proposed in, e.g., JP-A-2-209977, JP-A-3-206458 and JP-A-2-19847. Although those compositions are suitable for the case of using the KrF excimer laser beam of 248 nm, they have low sensitivity to an ArF excimer laser beam because their absorbance at the wavelength of ArF excimer laser beam is substantially too high. This drawback is accompanied with other drawbacks of causing deterioration in, e.g., definition, focus latitude and pattern profile. Therefore, it is necessary for those compositions to be improved in many points.
As photoresist compositions suitable for the ArF excimer light source, the combinations of (meth) acrylic resins showing less absorption of light than partially hydroxylated styrene resins with compounds capable of generating acids upon exposure are proposed in, e.g., JP-A-7-199467 and JP-A-7-252324. Specifically, JP-A-6-289615 discloses the acrylic resin having carboxyl groups the oxygen atoms of which are bonded to tertiary carbon-containing organic groups to form ester linkages.
Further, JP-A-7-234511 discloses the acid-decomposable resin containing both acrylate and fumarate as constitutional repeating units. However, those resins cannot provide satisfactory pattern profile and adhesiveness to a substrate. In other words, they are not successful in ensuring satisfactory resist properties.
Furthermore, the photoresist compositions proposed for the purpose of improving dry etching resistance, sensitivity and so on use resins into which alicyclic hydrocarbon groups are introduced, as the resins to be combined with compounds capable of generating acids upon exposure to light. For instance, the photoresist composition using a resin having alicyclic hydrocarbon moieties and acrylate, fumarate, acrylonitrile, maleimide or maleic anhydride moieties in its main chain is proposed in JP-A-10-10739. In JP-A-9-325498 is proposed the photoresist composition using a resin containing norbornane or cyclohexane moieties, which are each substituted with at least one acid group such as carboxylic group, as the alicyclic groups in the main chain. In addition, the photoresist compositions using resins containing norbornane moieties in their main chains are proposed in WO 97/33198 and European Patent 0,789,278. However, those photoresist compositions encounter a new problem that the resist patterns formed have cracks in them; besides, they are insufficient in the adhesiveness to a substrate and cannot ensure a satisfactory profile in the resist pattern.